Incorporation of non-fibrous fillers having a particle or plate shape into organic polymers for improvement of characteristics is widely known. For example, addition of a nonfibrous filler to thermoplastic resins for reduction of molding shrinkage has been proposed, e.g., in JP-A-3-52953 and JP-A-4-361027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, it is also well known that this method is not so effective to improve mechanical characteristics or thermal characteristics.
Incorporation of fibrous fillers or reinforcing fiber in addition to the nonfibrous fillers, so-called hybridization of fillers, has been proposed as an approach to solution of the above problem as disclosed, e.g., in JP-A-60-63255 and JP-A-62-218445. Use of fibrous Xonotlite as a fibrous filler has been attempted for this purpose (as disclosed, e.g., in JP-A-2-88642).
Blending of a modifier, such as rubber, a thermoplastic elastomer or a rubber-modified thermoplastic resin, is known as a method for improving impact resistance of thermoplastic resins (as disclosed, e.g., in JP-A-56-118456, JP-A-61-192752, and JP-A-62-68851). However, it is also known that this method incurs reduction in strength, elastic modulus, and heat resistance.
In order to eliminate the above problem, it has been proposed to incorporate reinforcing fiber, fibrous fillers, or particulate or plate-shaped nonfibrous fillers in addition to the above-mentioned modifier. Although the method of using reinforcing fiber (as disclosed, e.g., in JP-A-62-12745) is effective to improve mechanical characteristics, such as strength or elastic modulus, and heat resistance, the resulting molded articles suffer from poor appearance or anisotropy. Further, the method of using particulate or plate-shaped nonfibrous fillers (as disclosed, e.g., in JP-A-2-255761) fails to bring about sufficient improving effects on such characteristics as mentioned above while hardly causing poor appearance or anisotropy.
Hence, a combined use of nonfibrous fillers and fibrous fillers (hybridization of fillers) (as disclosed, e.g., in JP-A-4-318064) or a combined use of fibrous fillers and reinforcing fibers, such as glass fiber and carbon fiber (as disclosed, e.g., in JP-A-4-288341), has been attempted. Among the fibrous fillers meeting this purpose is included fibrous Xonotlite (as disclosed, e.g., in JP-A-61-69848 and JP-A-3-45652).
Fibrous Xonotlite as referred to above is well known as a reinforcement for resins and rubbers (as disclosed, e.g., in JP-A-48-19498, JP-A-50-52145, JP-A-53-30499, and JP-A-53-33245).
However, conventional fibrous Xonotlite has poor dispersibility in a matrix due to its liability to agglomeration and, in addition, exhibits insufficient wettability by a matrix-forming organic polymeric substance. It follows that the resulting composite material fails to display the desired physical properties.
For the purpose of improving surface properties of fibrous Xonotlite, surface treatment with a coupling agent, etc. has been attempted (as disclosed, e.g., in JP-A-50-158647, JP-A-50-158648, JP-A-51-7055, and JP-A-4-108855), but satisfactory results have not obtained yet.
JP-A-2-284940 discloses a flame retardant resin composition comprising polypropylene, glass fiber, fibrous magnesium hydroxide and/or fibrous Xonotlite, and an acid-modified polyolefin at a specific blending ratio, in which the fibrous Xonotlite has an average fiber diameter of 0.1 to 3 .mu.m and an aspect ratio of 10 to 200.
In JP-A-2-284940, however, no consideration is given to the adhesion between fibrous Xonotlite and the matrix resin, which influences on the reinforcing effects of the fibrous Xonotlite. The problem associated with this technique is that when microcrystalline fibrous Xonotlite having a small average fiber diameter, or a short average fiber length, or a small average fiber diameter with a short average fiber length is used, such fibrous Xonotlite not only has a strong tendency to agglomeration but also is highly bulky so that it is difficult to apply as a reinforcement for resins.
In order to settle the above problem, the inventors of the present invention previously proposed specific calcium silicate hydrate (i.e., fibrous Xonotlite) and a process for producing the same as disclosed in JP-A-6-128412, characterized in that the fibrous Xonotlite used has a BET specific surface area of not less than 21 m.sup.2 /g as measured by nitrogen adsorption and has been subjected to surface treatment with a surface active agent and/or a coupling agent and shaped into granules. The fibrous Xonotlite disclosed is suited for use as a reinforcement for various resins and molded articles thereof.
According to JP-A-6-128412, as having a BET specific surface area of not less than 21 m.sup.2 /g as measured by nitrogen adsorption, the fibrous Xonotlite exhibits high adhesion to a matrix resin so that it exerts high reinforcing effects on a matrix resin even if it is microcrystalline as in JP-A-2-284940 supra. Further, since it has been treated with a surface active agent and/or a coupling agent and has a granular shape, it is less bulky and highly fluid and thereby easy to knead together with a resin. The technique thus achieved improvements over the conventional techniques. However, no particular consideration being given to the shape of the fibrous Xonotlite, the following problems still remain unsolved. When fibrous Xonotlite having a small average fiber diameter or a long average fiber length is used, if kneading force is weak, the fibers cannot be sufficiently dispersed in the resin and, if kneading force is strong, the fibers are apt to be broken. When fibrous Xonotlite having a large average fiber diameter or a short average fiber length is used, the reinforcing effects, that is, effects of improving mechanical strength are not sufficient.